Strongback design????

[Jack D Davis]

In planning construction of a welded aluminum fishing boat (<15', design not decided upon yet), I envision a strongback similar to this. I designed it to rotate for easy access to weld joints. The whole thing would be made from steel tubing and bolted to the concrete floor of my shop, under my jib crane so that it could be easily moved after I'm done on the strongback. The drawings aren't dimensioned since I imagine the final details would depend on the chosen boat plan. Does this make sense? Am I on the right track?

[Wynand N]

Hi Jack D

No problems with your strongback design. It will work like a charm with a smallish 15ft Alu boat on it.
Nice CAD graphics.

Although the following pics are not strongbacks in the sense, I feel there is some relevance in showing them. I had built a 57ft that was to be NDT on hull welds - filmed X rayed- and I went the TIG way welding the hull. The problem was how to turn the hull so we always have a "downhand" position whil'st zapping her with the TIG process.
What you see was the end result end it worked like a charm. The whole boat "floats" in the air, attached to the hoops only, which run into stationary rollers. We installed some serious brakes and chainblocks to kept everything safe and secure. Can you imagine a 57 doing a 360 on land

[Jack D Davis]

Amazing! Does it travel as it's rolling? Or does it stay in place? Pic's are dark and I can't see much detail.

[SolomonGrundy]

Actually, it looks a little flimsey to me... how long and wide is it supposed to be? I would add a little cross bracing for added rigidity. Once that strongback is loaded with framing, longs, bottom and side sheets, the unsuported middle may sag some causing a "hogging" effect noticed as a hump in the sheer. The best small boat jigs (I won't even go into building right side up or upside down) I have seen are low to the ground, built from heavy tubular steel with tubular steel cross members. These are fitted with jack screw pads along the length of the jig: on all four corners, mid-jig and equidistant between the corners and mid-jig to facilitate leveling the jig with no sags.

[Jack D Davis]

As shown it is 2" x 3" (could be 4") x .120 steel tube. Rectangle is 3'x16'....... I doubt that it will sag much in 16' and spread is also doubtful with form(s) or frame(s) bolted or welded across.

Of course, I'm not sure if this (a strongback) is even necessary. I've been told I can build a small boat without a strongback, but I'm having trouble visualizing how this could be done. Especially, since I will be working alone.

[SolomonGrundy]

Jack, if you don't think that it will sag, that's your call. It seems to be supported only on 2 points at the ends on CL. The design almost looks free to rotate about the axis if it wasn't welded in there. I've built plenty of aluminum boats and I've never seen a strongback as such. They are called Strong-backs for a reason, they need to take the weight of the jig, if any, and the boat and remain quite rigid. What do the plans call for?

[Jack D Davis]

The design almost looks free to rotate about the axis

That's exactly right. I designed it to rotate for easy access for welding. I envision welding or bolting forms, and/or stations/jigs to the strongback for forming the longitudinals and hull plates. The round tube sections will be drilled for a pin, in the appropriate places to hold the hull at the desired angles. I'm talking about a 14'-15' boat hull that will weigh, maybe, 200# spread over the length of the strongback. The hull will be able to rotate a complete 360 deg. I wouldn't go to this much trouble for one boat, but I'm planning several on the same forms.

I've been unable to find suitable plans, so I'm designing my own with a lot of help from an experienced designer/builder.